No, the rack manufacturer says not to use it on a carbon seatpost. So do as they say, not as I do. I put inner tube under mine, the theory being the seatpost will be protected. And the rack itself has a limit of 20 or 25 pounds, and I don't over load it. Most of the time I doubt if I have five pounds of stuff in there.

I'm sure someone can explain why it's ok to have carbon cranks, with pedals, and carbon bars, with a stem, and a carbon seatpost, with a seat, but it's not ok to have a rack _on_ the seatpost. But I've put several thousand miles on it with that set up and haven't had a problem yet, and I guess if I do- then I'll switch to an aluminium post.